Catalyst and process for dehydrogenation and dehydrocyclization

ABSTRACT

A method for improving the activity of a dehydrogenation and dehydrocyclization catalyst composition and a process for improving the conversion and selectivity of paraffin hydrocarbons to olefin and aromatic compounds. The novel processes comprise adding calcium aluminate to a catalyst composition comprising a zinc aluminate support and a catalyst metal, and optionally a promoter metal.

This invention relates to catalytic dehydrogenation anddehydrocyclization of organic compounds. In another aspect, it relatesto dehydrogenation and dehydrocyclization processes. In yet anotheraspect, it relates to dehydrogenation and dehydrocyclization catalysts.

In recent years, the composition of the motor gasoline pool hasdramatically been altered due to changes in environmental regulations.Because of regulations limiting the allowable maximum gasoline vaporpressure, the amount of high vapor pressure components such as butanethat can be blended in gasoline must be reduced in order for finalgasoline products to meet the various vapor pressure requirementsdictated by regulation. Consequently, these gasoline vapor pressurelimitations have resulted in an increased supply availability of highvapor pressure butanes due to their removal from the gasoline pool.

In addition to the vapor pressure regulations, there have also been anumber of other regulations which have contributed to the reduction ofhigh octane components available for use in the gasoline pool. Forinstance, limits on the amount of lead additive which may be used ingasoline have resulted in the removal of substantial quantities ofoctane from the gasoline pool. Further proposed regulations, such as thelimitations on the quantity of high octane aromatic compounds that maybe mixed in gasoline, may also impact the amount of octane available foruse in the gasoline pool. There are numerous other factors contributingto the reduction in available octane.

One possible response to the changing composition of the gasoline poolis for industry to dehydrogenate low molecular weight alkanes to alkenesthat can be used as a feedstock in downstream processes, such as the HFalkylation and methyl tertiary butyl ether (MTBE) processes, by whichhigh octane, low vapor pressure gasoline blending components can beproduced. A further response is to dehydrocyclize low octane, lightparaffins to high octane aromatics. The production of MTBE, whichinvolves reacting the isobutylene produced from the dehydrogenationprocess with methanol, provides a source of oxygen in gasoline when usedas a gasoline blending component thereby helping to meet the variousregulations which may require the addition of oxygen compounds.Furthermore, by removing butanes from the gasoline pool anddehydrogenating them to form butenes that are used as feedstocks todownstream operations, high vapor pressure gasoline components areremoved from the gasoline pool and are replaced with low vapor pressure,high octane components.

There are numerous approaches known for the dehydrogenation of organiccompounds. One such approach is the non-catalytic thermaldehydrogenation of organic compounds. However, this method ofdehydrogenation has not been commonly accepted because of the extensiveundesirable side reactions and substantial coke production which takeplace. Thus, it has been sought to develop a catalytic dehydrogenationand dehydrocyclization process that provides a high conversion offeedstock and high selectivity to desirable end-products. To accomplishthis, a vital aspect of the process is the use of a catalyst havingcertain desirable properties. Some of these desired properties are thatthe catalyst have the ability to convert a large fraction of a givenfeed material to end-products and that the conversion be highlyselective in producing certain desired end-products. In using a catalystthat gives a high conversion per pass, energy costs associated with agiven process can be lowered by reducing the cost of separation andrecycling of the unconverted feed material. In addition, a highlyselective catalyst will improve the operating efficiency of the processby reducing the amount of unwanted end-products produced.

In the dehydrogenation and dehydrocyclization processes, considerableadvantages are obtainable when hydrocarbon feed to the reactor can bediluted with steam. The mixing of the hydrocarbon feed with steam hasthe effect of lowering the partial pressure of the hydrogen producedfrom the reaction and that of the hydrocarbon thus shifting theequilibrium conditions within the reactor toward greater conversion ofthe feedstock. Additional benefits from the use of steam are that it canprovide a portion of the heat of reaction required and it can retard therate of coke deposition on the catalyst. Furthermore, expensivecompression of products can be avoided since elevated pressures can beemployed and steam can be readily condensed after dehydrogenation iseffected. Because of the great advantages possible from using a steamdiluent in the dehydrogenation and dehydrocyclization of hydrocarbons,attempts have been made to develop catalysts which have high stabilityto steam and that allow dehydrogenation of alkanes in the presence ofsteam.

There are numerous other desirable catalyst properties which contributeto the improved performance and design of a dehydrogenation anddehydrocyclization processes. Among these is a catalyst having highcrush strength. The higher the crush strength of the catalyst the moredurable the catalyst and the greater the amount of pressure drop whichcan be experienced in a bed reactor without damaging the catalyst.Moreover, the useful life of a catalyst may be increased.

It is, therefore, an object of this invention to provide an improveddehydrogenation and dehydrocyclization process.

It is another objective of this invention to provide an improveddehydrogenation and dehydrocyclization catalyst.

I have discovered a novel method for improving the activity andselectivity of a catalyst composition used in the dehydrogenation anddehydrocyclization of steam-diluted hydrocarbons. It has been found anddemonstrated herein that it is a critical element of this invention tohave the presence of calcium aluminate in the catalyst support ofdehydrogenation and dehydrocyclization catalyst. The standard supportformulation of the prior art included a 100 percent zinc aluminatesupport, but, the composition was not suitable for commercial use. Ithas been found that new and material beneficial properties that aredifferent from those disclosed in the prior art are achievable by theincorporation of calcium aluminate into the support material of thisinvention. Unexpectedly, by the addition of calcium aluminate to adehydrogenation or dehydrocyclization catalyst supported by zincaluminate, the percent conversion of saturated hydrocarbons tounsaturated hydrocarbons is substantially improved. The presence ofcalcium aluminate as a support accounts for an overall improvement inactivity of the catalyst composition. Additionally, it has beendiscovered that the method herein results in a longer lived catalystactivity than those lives shown in the art.

The novel method for improving the activity of a dehydrogenation ordehydrocyclization catalyst is generally performed by combining with asupport composition comprising zinc aluminate and certain Group VIIImetals as a catalyst, calcium aluminate in any manner known to the art;and, optionally, certain Group IA, Group IIA, Group IIB, lead, tin,germanium, gold or silver as promoters of the activity of the supportedGroup VIII metal catalyst. This invention further includes a process forthe dehydrogenation and dehydrocyclization of hydrocarbon utilizing thenovel step of adding calcium aluminate to a catalyst composition for thepurpose of improving conversion and selectivity.

Other objects, aspects, and features of the present invention will beevident from the following detailed description of the invention, theclaims and the drawings in which:

FIG. 1 is a graphical diagram comparing propane conversion as a functionof temperature for three different catalyst compositions, which includesthe novel catalyst composition of the present invention.

FIG. 2 is a graphical diagram comparing propylene selectivity as afunction of temperature for three different catalyst compositions, whichincludes the novel catalyst composition of the present invention.

FIG. 3 is a graphical diagram comparing propylene selectivity as afunction of conversion for three different catalyst compositions, whichincludes the novel catalyst composition of the present invention.

The catalyst activity and selectivity of this invention can be improvedby combining in any manner known to the art, certain Group VIII metalsor metal compounds capable of reduction to the metal and mixtures of twoor more thereof and zinc aluminate with calcium aluminate. As usedherein, the term Group VIII metals, or similar language, specificallyinclude iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium,iridium, and platinum. Platinum, which is very effective, is preferred.The Group VIII metal content of the catalyst can be in the range of fromabout 0.01 to about 5 weight percent of the support and, in a preferredembodiment, it is in the range of from about 0.1 to about 1 weightpercent of the support. Throughout this application the term "weightpercent of the support" means parts by weight per 100 parts by weight ofsupport material.

Any platinum group metal compound that produces the desired results canbe used. In the discussion of the compounds that can be used, theplatinum compounds will be used as nonlimiting examples. It is to beunderstood that similar compounds of the other platinum group metals canbe used. Examples of simple or noncoordination compounds that can beused are platonic chloride, chloroplatinic acid, ammoniumchloroplatinate, and the like. Nonlimiting examples of coordinationplatinum compounds that can be used are: platinum amminoacetate,platinum dimethyl dioxime, tetraammineplatinum hydroxide, platinumdiammine dinitrate, platinum tetraammine dihydroxide, platinum diamminedihydroxide, platinum hexammine dihydroxide, platinum hexamminetetrahydroxide, platinum diammine tetrahydroxide, platinum diamminedihydroxide dinitrate, platinum diammine tetranitrate, platinum diamminedinitrite, platinum tetraammine dicarbonate, platinum diammine oxalate,and the like. Additionally, many complex or coordination divalent andtetravelent platinum compounds are known and can be used.

When added to the support by impregnation from solution, some of thecompounds can be added from aqueous solution, but others will requirenon-aqueous solvents such as alcohols, hydrocarbons, ethers, ketones andthe like.

In addition to the Group VIII metals, the catalyst composition cancontain a tin group metal including lead, tin, germanium and mixtures oftwo or more thereof as a promoter. The tin component can be depositedwith the Group VIII metal component upon the catalyst support,separately or together, by any manner known in the art such as bydeposition from aqueous and non-aqueous solution of tin halides,nitrates, oxalates, acetates, oxides, hydroxides and the like. The tingroup metal can exist in the range upwardly to about 5 weight percent ofsaid support and, in a preferred embodiment, it can exist in the rangeof from about 0.1 to about 1.5 weight percent of said support.

Although any tin group metal in compound form is fully within the scopeof this invention, some convenient tin group compounds are the halides,nitrates, oxalates, acetates, carbonates, propitiates, tartrates,bromates, chlorates, oxides, hydroxides, and the like of tin, germaniumand lead. Tin, itself, is the preferred tin group metal and impregnationof the catalyst support with tin compounds such as the stannous halidexis particularly effective and convenient.

Also, in addition to the Group VIII metals, the catalyst composition canfurther include, with or without the tin group metal, a Group IA orGroup II metal or metal compound as a promoter. This can be convenientlydone by conventional impregnation. The amount of each alkali metalcompound or combination of compounds can exist in the range upwardly toabout 5 weight percent of the support; however, in a preferredembodiment, a range from about 0.1 to about 1 weight percent of saidsupport is used. Convenient compounds which can be used are thecarbonates, acetates, and hydroxides and the like of sodium, barium,potassium, calcium, and the like.

Another promoter metal which can be used in an embodiment of thisinvention, is a metal selected from the group consisting of gold, silverand mixtures thereof. This promoter metal may or may not be used incombination with the tin group promoter metals, Group IA, or Group IIpromoter metals. The amount of gold, silver or mixtures of such to beused as a promoter is in the approximate range of from about 0.1 toabout 5 weight percent of the support. Suitable gold and silvercompounds include, but are not limited to, arsinic chloride, arsinicsulfate, aurous chloride, tetrachlorauric acid, silver nitrate, silveracetate, silver cyanide and the like.

The support material of this invention comprises a mixture of zincaluminate and calcium aluminate. The support can be prepared by anymethod known in the art. While the prior art teaches that the mosteffective support composition is zinc aluminate, the novel method ofthis invention gives a catalyst, when included with the Group VIIImetals or promoter metals, or both, having greater activity andselectivity than those of a catalyst using exclusively a zinc aluminatesupport. A further advantage from having a catalyst support mixture ofzinc aluminate and calcium aluminate is that the addition of calciumaluminate improves the crush strength of the base material.

Any suitable amount of calcium aluminate can be present in the supportmaterial. In a preferred embodiment, calcium aluminate is added so as tobe present in the range of from about 5 weight percent to about 25weight percent of the support. Most preferably, the content of calciumaluminate is in the range of from about 10 weight percent to about 18weight percent.

The 25 weight percent content limitation for the presence of calciumaluminate appears to be a critical limit for this component of the novelcomposition. As the presence of calcium aluminate increases, it becomesincreasingly more difficult to regenerate the composition once it hasbecome spent; however, the greater the proportion of the compositionthat is calcium aluminate the greater the activity and selectivity ofthe composition. Additionally, the crush strength of the composition isimproved with increasing amounts of calcium aluminate present. Whenaccounting for these newly discovered beneficial properties of thecomposition that are obtainable by the addition of calcium aluminatealong with the negative effects associated with the addition of calciumaluminate, the 25 weight percent limitation appears to be close to themaximum permisible amount of calcium aluminate that can be present whilestill giving a composition having the desireable properties of improvedcrush strength and improved catalyst activity and selectivity.

Improved dehydrogenation and dehydrocyclization processes are achievedby the use of the novel step of adding calcium aluminate to acomposition comprising zinc aluminate and a catalyst metal prior to acontacting step. In these processes, paraffins containing five carbonsor less are dehydrogenated to the respective olefin compounds, andparaffins containing six or more carbon atoms can be dehydrocyclized tocyclic and aromatic compounds or they can be dehydrogenated to olefincompounds. In a preferred embodiment of this invention, paraffinhydrocarbons are first preheated and vaporized and mixed with steam withthe thus formed mixture being passed over a bed of the novel catalystcomposition of this invention. The mole ratio of steam mixed with thehydrocarbon can be in the range of from about 2 to about 30 moles ofsteam per mole of hydrocarbon; preferably, however, the mole ratio willbe in the range of from about 2 to about 10. The presence of steam as adiluent provides a benefit by reducing the partial pressure of thehydrocarbons and hydrogen present in the reactor and thereby shiftingthe equilibrium conditions within the reactor toward greater conversionof the hydrocarbons.

A preferred approach to utilizing the inventive process is to pass thevaporized, steam diluted hydrocarbon through either a single or aplurality of fixed bed tube reactors. Because the dehydrogenationreaction is generally an endothermic reaction, to maintain a nearisothermal reaction, heat must be added. It has been found that the mostfavorable reaction kinetics can be achieved by operating the reactornon-adiabatically. In this mode of operation, it is preferred that thereactor be of the tubular type with the heat source being external tothe tubes of the reactor which may, for example, be the firebox of a gasfired heater. Any number of tube reactors may be used, but it ispreferred that a multiplicity of tubes be used where one or more tubesmay be removed from service for the purpose of regeneration of thecatalyst simultaneously while the other tubes remain in operation. Onedesign configuration using the novel process of this invention is tohave eight reactor furnaces each of which is associated and operated inconjunction with a reactor section. Each reactor section can include aplurality of as many as 150 individual reactor tubes. Of the eightreactor sections, it is preferred that seven of the reactor operatingsections be in operation while one of the reactor sections issimultaneously undergoing regeneration.

The dehydrogenation reactor operating conditions are set so as tooptimize the process by taking into account such factors as the type offeedstock being processed, operating costs, product values, and productyields. Typically, it is advantageous to feed the reactors at a ratewhich gives a liquid hourly space velocity (LHSV) ranging from about 0.5to about 10 volumes of liquid hydrocarbon feed per hour per volume ofcatalyst. For computing the value for liquid hourly space velocity, thevolume of liquid hydrocarbon is determined at standard conditions of 60°F. and atmospheric pressure, and the volume of catalyst is determined bythe volume of the catalyst contained within the reactor vessel. Thereactor pressure can range from below atmospheric pressure to about 300psia. It is preferred, however, to minimize the operating pressure ofthe reactor in order to improve conversion, but an advantage fromoperating the reactor at higher operating pressures is that thecompression ratio of the downstream reactor effluent compression can beminimized by a higher operating pressure thereby providing certaindesign and operating benefits. The optimum operating pressure isdetermined by taking into account all these considerations. As for thereactor temperature, it can range from about 900° F. to about 1150° F.depending upon the type of hydrocarbon being processed and otherconstraints. Generally, the higher the operating temperature the greaterthe conversion.

The reactor effluent is passed through a feed/effluent heat exchanger ora series of feed/effluent heat exchangers in which heat contained withinthe reactor effluent stream is exchanged with incoming hydrocarbon feedthat is being charged to the tube reactors. After passing through thefeed/effluent exchangers, the reactor effluent is passed through a steamsection in which superheated steam is produced for use in a steamexpander. The reactor effluent leaving the steam generation section isoptionally passed through a reboiler exchanger followed by an exchangeof heat with incoming hydrocarbon feed in the feed preheat section. Theuse of a reboiler exchanger will depend upon the feedstock to theprocess. The process effluent is further transferred through a series ofphase separators and heat exchangers where condensate is separated fromthe hydrocarbons and uncondensed steam and is returned to the steamgeneration section for reuse. The final vaporous product effluent iscompressed prior to being charged to a recovery system where the finalolefin and aromatic products, light ends, and hydrogen are recovered. Inthe recovery system, hydrogen gas can optionally be recovered by anysuitable means, including, for example, separation membranes, leanoil/rich oil systems, cryogenic processes and pressure swing absorptionsystems, depending upon the economic value of the hydrogen and itspotential downstream uses. The light ends recovered and any unrecoveredhydrogen can be used as a fuel source in the reactor furnace. The finalend-product is treated and sent to storage or to other downstreamprocesses.

Other objects, aspects, and features of the present invention will beevident from the following example.

EXAMPLE I

Two catalyst formulations for dehydrogenation or dehydrocyclization, orboth, were prepared for pilot plant testing and for comparison withdehydrogenation catalyst having essentially a 100 percent zinc aluminatesupport. Presented in Table I are the compositions of the two catalyststested and the analysis of the 100 percent zinc aluminate comparisoncatalyst. Catalyst A was formulated by mixing 10 percent calciumaluminate and 90 percent zinc aluminate, and catalyst B was formulatedby mixing 18 percent calcium aluminate and 82 percent zinc aluminate.The calcium aluminate used in catalyst A was a commercially producedproduct of Lone Star Lafarge, Inc. known by its tradename Secar® 71calcium aluminate. The calcium aluminate used in catalyst B was acommercially produced product of Alcoa® known by its tradename CA-25calcium aluminate. The two calcium aluminates are produced by differentmanufacturing techniques and have slightly different compositions whichmay account for some of the differences in the properties betweencatalyst A and catalyst B as hereinafter is described. The zincaluminate used in all catalyst was prepared by mixing zinc oxide (ZnO)with a fumed alumina (Al₂ O₃), which is manufactured and is commerciallyavailable from Degussa Corporation as its product known as aluminumoxide C, and calcining the mixture at a temperature of about 1550° F.for a period of five hours. Each catalyst base was impregnated byconventional means with platinum metal catalyst and a tin metalpromoter. The resulting catalyst was formed into 4-8 mesh catalystpellets and packed into a two-inch diameter reactor equipped withthermocouples for measuring the reactor bed temperature and forobtaining an axial and a radial temperature profile within said bed.

                  TABLE I                                                         ______________________________________                                                             Catalyst A                                                                              Catalyst B                                                 Comparison                                                                             (10% Secar                                                                              (18%                                                       Catalyst 71)       CA-25)                                                     100% Zinc                                                                              Calcium   Calcium                                                    Aluminate                                                                              Aluminate Aluminate                                      ______________________________________                                        Aluminum (Al)            28.5      28.1                                       Calcium (Ca)             1.3       2.0                                        Zinc (Zn)                33.0      30.0                                       Platinum (Pt) 0.6        0.51      0.55                                       Tin (Sn)      1.0        1.0       0.9                                        Pore Volume cc/gm        0.54      --                                         Surface Area m.sup.2 /g  29.0      31.5                                       Skeletal Density g/cc    4.29      4.29                                       Crush Strength (lb)                                                                         7.2        28.6      16.5                                       Mercury                                                                       Poresimetry Data:                                                             Pore Area m.sup.2 /g     71.0      89.4                                       Ave Pore Diam A          406       208                                        Bulk Density g/cc        1.59      1.51                                       Skeletal Density g/cc    5.55      5.12                                       Median Pore Diameter     406       343                                        (Vol) A                                                                       Median Pore Diameter     169       131                                        (Area) A                                                                      ______________________________________                                    

A series of propane dehydrogenation tests were performed using thecatalyst described in Table I. In performing the test runs, superheatedsteam at a temperature of approximately 1300° F. was mixed with propanehaving an approximate temperature of 600° F. The ratio ofsteam-to-hydrocarbon was maintained at approximately a 4 to 1 ratio.Prior to charging this mixture to the reactor, it was passed through afinal trim heater to bring the temperature up to the desired reactiontemperature. The volumetric charge rate to the reactor was set so as togive a liquid hourly space velocity (LHSV) of 4 volumes of hydrocarbonfeed per volume of catalyst per unit time with the time unit as hours.The reactor pressure was maintained at 50 psig by a pressure controlvalve placed at the outlet of the reactor. The reactor temperature wasset at various temperatures to generate conversion and selectivity dataof propane to propylene for comparison with the different catalystcompositions.

FIGS. 1, 2 and 3 are provided to graphically present the test runresults and show a comparison of the test results for the novel catalystcompositions with the standard 100 percent zinc aluminate supportcatalyst. The solid lines on each FIG. represents the predictedperformance of the standard catalyst based upon an empirical statisticalmodel developed from data generated from eighty-three test runs usingthe 100 percent zinc aluminate support catalyst. The data points on eachFIG. show the test run results for Catalyst A and for Catalyst B. FIG. 1shows the percent conversion of propane as a function of average reactorbed temperature for each of the three catalysts tested. As can be seenfrom the data, conversion increases with increasing temperature. Therelationship between propane selectivity to propylene and reactortemperature for each of the three catalysts is presented in FIG. 2. FIG.2 shows that, generally, at a given reactor temperature, Catalyst A andCatalyst B both give a higher percent selectivity than the standardcatalyst. FIG. 3 shows the relationship between the selectivity of eachof the three catalysts and conversion. Generally, at a fixed percentpropane conversion, both Catalyst A and Catalyst B give a higher percentselectivity than the standard catalyst.

The test results unexpectedly show that the selectivity of thedehydrogenation of propane to propylene can be improved by using acatalyst having a mixture of calcium aluminate and zinc aluminatesupport rather than the traditional catalyst support mixture of 100percent zinc aluminate. The catalytic selectivity improves as the amountof calcium aluminate is increased, however above about 25 weight percentcalcium aluminate, the catalyst becomes increasingly difficult toregenerate. It is believed that catalyst A and B represent near optimummixtures balancing improved performance with suitable regenerationcharacteristics.

Reasonable variations and modifications may be made in the combinationand arrangement of parts or elements or in the processes as heretoforset forth in the specification and shown in the drawings withoutdeparting from the spirit and scope of the invention as defined in thefollowing claims.

That which is claimed is:
 1. In a process wherein hydrocarbons arecontacted with a catalyst composition comprising a zinc aluminatesupport and a metal catalyst selected from the group consisting of GroupVIII metals, the improvement comprising:adding calcium aluminate to saidzinc aluminate support in the amount in the range of from about 5 toabout 25 weight percent, based upon the total combined weight of saidzinc aluminate and said calcium aluminate; and recovering a reactoreffluent product.
 2. A process as recited in claim 1, furtherincluding:preheating said hydrocarbons prior to contacting with saidcatalyst composition.
 3. A process as recited in claim 2, furtherincluding:mixing said preheated hydrocarbons with steam prior tocontacting with said catalyst composition.
 4. A process as recited inclaim 3, further including:cooling said reactor effluent to form acondensate phase and a vapor phase, and separating said condensate phaseand said vapor phase.
 5. A process as recited in claim 4, furtherincluding:compressing said vapor phase.
 6. A process as recited in claim5, further including:recovering a hydrogen product, a light ends productand a hydrocarbon product from the thus compressed vapor phase.
 7. Aprocess as recited in claim 1 wherein:said hydrocarbons comprisecompounds having from 3 to 8 carbon atoms.
 8. A process as recited inclaim 1 wherein:said contacting step is performed in a temperature rangefrom 900° F. to 1150° F.
 9. A process as recited in claim 1 wherein:saidcontacting step is performed at a pressure from below atmosphericpressure to 310 psia.
 10. A process as recited in claim 1 wherein:saidhydrocarbons are mixed with steam at a mole ratio of from 2 to 30 priorto contacting the thus formed mixture with said catalyst composition.11. A process as recited in claim 1 wherein:said hydrocarbons arecontacted with said catalyst composition at a rate to give a liquidhourly space velocity of from about 0.5 to about
 10. 12. In a processwherein hydrocarbons having from 3 to 8 carbon atoms are contacted witha catalyst composition comprising zinc aluminate support and a metalcatalyst selected from the group consisting of Group VIII metals andmixtures of two or more thereof, the improvement comprising:addingcalcium aluminate to said catalyst composition in the amount of about 10to 18 weight percent, based upon the total combined weight of said zincaluminate and said calcium aluminate, wherein said hydrocarbons areadmixed with steam in a mole ratio of steam to hydrocarbon in the rangeof from about 2 to about 30 at dehydrogenating conditions wherein thetemperature range is of from about 900° F. to about 1150° F., thepressure range is of from below atmospheric pressure to about 300 psia,and the liquid hourly space velocity is in the range of from about 0.5to about 10.